The present invention relates generally to lithography and, more particularly, to micro-surface fabrication.
Many integrated optics, semiconductor, and micro-machining applications require the creation of well-controlled, three-dimensional surfaces on a sub-micron scale. These applications include micro-optical system element (mirror, lens, waveguide, etc.) fabrication, particularly where dense arrays are required. Often the surface configuration is defined by a mathematical function to facilitate fabrication. There is a need for continuous control of the surface configuration to match a predefined mathematical function tailored to a specific application.
Formation of well-controlled surfaces on a sub-micron scale has many potential applications in both semiconductor manufacturing and micro-machining. A semiconductor example is the formation of tapered vias, in which the taper is useful in optimizing subsequent fill characteristics or relieving stress buildup at the substrate interface. A micro-machining example is the ability to manufacture optical components with arbitrary, yet well-controlled, angle and curvature.
Current methods of creating surfaces are limited by their ability to control the surface contours on a sub-micron scale and do not enable rapid replication of a given surface profile. These methods include various etching and polishing techniques. Standard lithography is one such process.
Standard lithography does not enable continuous control of surface configuration. For an example of lithography on discrete layers, see U.S. Pat. No. 4,944,838 to Koch. For an example of lithography and mold transfer, see U.S. Pat. No. 5,230,990 to Iwasaki. For an example of lithography and etch transfer with thermal processing, see U.S. Pat. No. 5,079,130 to Derkits. For an example of lithography in absorbing resist, see J. P. Kirk and M. S. Hibbs, xe2x80x9cDUV diagnostics using continuous tone photoresist,xe2x80x9d 1463 SPIE at 575 (1991).
X-ray lithography and electroplating also do not enable continuous control of surface configuration. For example, see U.S. Pat. No. 5,190,637 to Guckel.
Ablation and ion milling are relatively slow processes. It is difficult to control replication in such processes, and they have relatively poor resolution. Therefore, ablation and ion milling are not compatible with standard lithography techniques. For an example of X-ray ablation of absorbing material, see U.S. Pat. No. 5,730,924 to Katoh. For an example of laser ablation using mask to produce multiple beams, see U.S. Pat. No. 4,128,752 to Gravel.
The deficiencies of the conventional methods for creating surfaces show that a need still exists for formation of well controlled surfaces. To overcome the shortcomings of the conventional methods, a new lithography method is provided. An object of the present invention is to control creation of a microscopic three dimensional surface.
To achieve these and other objects, and in view of its purposes, the present invention provides a lithographic process to fabricate a microscopic, three-dimensional surface. The surface is defined by a mathematical function using a binary mask, consisting partly or wholly of subresolution features, and a photoresist film of pre-specified absorption and thickness. The process comprises the steps of (a) creating a mask, (b) imaging the mask pattern on an absorbing photoresist film to a predetermined thickness, and (c) transferring the three dimensional surface to a substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.